Minimally invasive, surgical techniques are becoming more prevalent as a means to reduce injury to the patient during surgical procedures, and thereby improving patient recovery to this ted, computer assisted surgical (CAS) techniques are being employed more frequently.
In conventional CAS, surgeons employ computational and image processing technologies to assist in surgical procedure. High resolution, three-dimensional internal images of a patient are taken prior to surgery, for example, computerized tomograph (CT) or magnetic resonance imaging (MRI). The images are digitized, processed and saved in a computer system for use in a variety of purposes. In this specification, CAS includes surgical planning, surgical navigation, image guided surgery, and the like.
For example, before surgery, the saved images are registered to the patient. During surgical procedure, the intra-operative position of surgical instruments connected to a CAS system may be tracked by positioning sensors. The position of tracked surgical instruments in space is computed and the information merged with the saved images of the patient. The computer displays the position of surgical instruments corresponding to the saved images of the patient. The images displayed are then updated in accordance with the positioning of tracked surgical instruments to provide the surgeon with a real-time view of she surgical instruments and the surgical site.
More recently, an intra-operative imaging modality incorporated into CAS for orthopedic surgical procedure and navigation is fluoroscopy. Fluoroscopy utilizes X-ray radiation to obtain pre-operative internal images.
Generally, fluoroscopy utilizes a C-arm x-ray imaging device. A typical C-arm imaging device consists of a C-arm attached to a base with an X-ray source at one end of the C-arm and an image intensifier on the other end. The X-ray source emits X-rays, which are passed through a patient's body. On the other side of the C-arm, the image intensifier detects the X-rays and converts the received photons into a video signal of a two-dimensional image. By taking multiple two-dimensional images from multiple perspectives, a three-dimensional image can be derived. To change the image perspective, the C-arm is rotated to multiple positions and X-ray radiation passed through the interested portion of the patient's body at various angles.
However, initial images generated exhibit distortion due to a number of sources. One source of distortion is gravity. As the C-arm is rotated about the measuring field, the force of gravity deforms the C-arm, resulting in a change in the distance between the X-ray source and the image intensifier. Such deformations vary with changes in the orientation of the C-arm, resulting in radial and rotational distortions of the image produced by the image intensifier.
Another source of distortion is the Earth's magnetic field. The Earth's magnetic field varies continuously. Changes in the magnetic field affects electron velocity in an image intensifier producing rotational distortion of the image that varies non-linearly in the radial direction.
Before the saved image may be employed for CAS, including image-guided surgery and surgical navigation, the system must compute substantially all distortion to allow the positioning information to be accurately overlaid on the saved image.
A calibration process is needed to characterise the C-arm to remove image distortion and to define a mathematical projection model that will allow the projection of tracked surgical instruments in the fluoroscopic images.
Currently existing C-arm calibration techniques use either one or two calibration plates provided adjacent to the image intensifier. The plates contain radio-opaque beads spaced in a well-defined geometry in one or more planes, and are positioned in the path of the X-rays. The beads are visible in the captured images. All systems have at least one grid plate mounted just above the X-ray receptor plate. This grid is often termed the “dewarp grid” since it serves to unwarp the image of both magnetic distortions and other artefacts of the image amplification process. The amount of distortion for each point in the image can be determined, because the true relative position of the beads in the initial images is known. The computer system can compute and then digitally compensate for distortion and generate a substantially distortion-free image.
Most systems have a second grid termed the “projection grid”. The projection grid lets the system calculate the X-ray projection lines and position of the X-ray source. The systems track the relative position of the C-arm intensifier and the surgical tools. Knowing the 3D position of the fiducial beads of the grid(s) and their 2D projections allows the system to calculate instrument position on each acquired image. The dewarp grid and the projection plate are each typically housed in plates that are termed the “calibration plate(s)”.
However in these systems, calibration plate(s) need to remain on the C-arm throughout the entire surgical procedure. The presence of the projection grid plate decreases the available space between the image intensifier and the X-ray source in which to position a patient, an operating table and instrumentation, and, as well, to conduct surgical procedure. The presence of fiducial beads in the image also degrades the image quality. As such, it is desirable to use a C-arm with only one dewarp grid, or more advantageously, without having to use any grids at all.
The main disadvantage associated with the use of a single dewarp grid, is the lack of information relating to C-arm deformation due to gravity-induced bending. This information is required to compete the C-arm source position.
PCT/CH97/00418 filed Nov. 4, 1997, and published May 14, 1999, Hofstetter, R. et al., discloses an optically tracked body aligned with the local gravitational field suspended from a stationary object in a operating room. The body's spatial coordinates are determined using position sensors in the room. A plotting unit, also in the room, determines a system of coordinates, one of whose axes corresponds with the orientation in the local gravitational field, which can be used as a reference system for use in computer-assisted surgery and navigation, and during image-guided surgery. A limitation associated with this method is that the suspended body must not move throughout the surgical procedure. If the suspended body or the positioning sensor moves, the calibration process needs to be repeated, which may cause some difficulty if calibration is required during a surgical procedure. Another limitation is that the sensor must be placed so that it can be men which is cumbersome during surgery.
Another method of calibration involves the use of encoders on a position sensor stand to relate position measurements to gravity, defined by the base of the stand. The encoders are calibrated in optical tracking space to obtain a gravity reference, which can be later used in image guided surgery and surgical navigation procedures. Limitations associated with this method include the necessity of encoding, of position sensors, the complexity of calibrating encoders and optical position sensors and the assumption that the position sensor base is oriented consistently with gravity.
Further, this method requires a dewarp grid. Without a dewarp grid, there is insufficient information to compute and correct for magnetic field induced distortions in the image.